The present disclosure relates to a light guide plate used in an edge-light-type backlight unit, a surface-emitting apparatus and liquid crystal display apparatus equipped with the light guide plate, and a method of producing a light guide plate.
A liquid crystal display (LCD) apparatus can realize low power consumption, miniaturization, and thinning as compared to a cathode ray tube (CRT), and those of various sizes are currently widely used in apparatuses ranging from small-size apparatuses such as a cellular phone, a portable game device, a digital camera, and a PDA (Personal Digital Assistants) to a large-size liquid crystal television.
Liquid crystal display apparatuses are categorized into a transmission type, a reflection type, and the like. In particular, a transmission-type liquid crystal display apparatus includes a liquid crystal display panel and a backlight unit as an illumination light source. As backlight units, there is an edge-light type in addition to a direct type in which a light source is disposed right below a liquid crystal display panel. The edge-light-type backlight unit is constituted of a light guide plate disposed on a back surface of the liquid crystal display panel, a light source disposed on a side surface of the light guide plate, a reflector plate that covers a surface on the other side of a light-emitting surface of the light guide plate, and the like.
Conventionally, for a light source used in those types of backlight units, a cold cathode fluorescent lamp (CCFL) that emits white light is widely used. Particularly in recent years, a backlight unit that uses a light-emitting diode (LED) as a light source is highly expected to be used for a mobile display of a cellular phone and the like.
In the edge-light-type backlight unit, light that has been emitted from a light source enters a light guide plate from an incident surface of the light guide plate and propagates inside the light guide plate while repeating a total reflection on an emitting surface and a reflecting surface of the light guide plate. During the propagation, light is diffused by a diffusion pattern formed on the reflecting surface and emitted from the emitting surface when an incidence angle with respect to the emitting surface becomes a critical angle or less, thus becoming illumination light of the liquid crystal display panel. To obtain optical characteristics as described above, a design of the light guide plate is optimized.
On the other hand, in the edge-light-type backlight unit, a light use efficiency and an in-plane luminance distribution are required to be improved. Specifically, it is necessary to cause light emitted from the light source to efficiently enter the light guide plate and prevent light that has propagated inside the light guide plate from leaking outwardly. Moreover, when an LED is used as the light source, light emitted from the light source is propagated inside the light guide plate while spreading like a fan from the incident surface of the light guide plate. Consequently, an area where light is not propagated is apt to be caused inside the light guide plate, with the result that it is difficult to uniformly emit light from the emitting surface of the light guide plate by merely optimizing a design of the light guide plate.
For solving the problem described above, Patent Document 1, for example, discloses a technique of roughening an incident surface of a light guide plate so that a light diffusion at the incident surface is used for reducing luminance unevenness. In addition, Patent Documents 2 and 3 disclose techniques of providing prisms or curved asperities on a light-incident surface of a light guide plate to reduce luminance unevenness.    Patent Document 1: Japanese Patent Application Laid-open No. 2001-243825    Patent Document 2: Japanese Patent Application Laid-open No. 2002-196151    Patent Document 3: Japanese Patent Application Laid-open No. 2005-228718
In the light guide plate disclosed in Patent Document 1, since light from the light source is forcibly diffused nondirectionally at a time the light enters, a traveling direction of the incident light also becomes nondirectional, and the light is diffused not only in a surface direction of the light guide plate but also in a thickness direction. As a result, light that is supposed to travel inside the light guide plate by a total reflection exits the light guide plate from the side surfaces or the back surface, and a light use efficiency is therefore lowered, which is a problem.
Further, in the light guide plates of Patent Documents 2 and 3, the light-incident surface is not flat since prisms or curved asperities are formed thereon, thus leading to a problem that light cannot enter the light guide plate efficiently due to a refraction or reflection of light at the concavoconvex portions. Therefore, also with this structure, it is difficult to improve a light use efficiency and a luminance distribution.
Furthermore, since there is a demand for additional thinning of a liquid crystal display apparatus in a field of a portable information terminal typified by a cellular phone or a portable game device in recent years, thinning of a light guide plate may become an essential agenda. However, a light incidence efficiency with respect to the incident surface lowers along with thinning of the light guide plate, and it becomes increasingly difficult to improve a light use efficiency and luminance distribution characteristics.
Therefore, it is desired to provide a light guide plate, a surface-emitting apparatus, a liquid crystal display apparatus, and a method of producing a light guide plate with which a light use efficiency and luminance distribution characteristics can be improved and that can cope with thinning.